Monogenetic, nested volcanic complexes (e.g. Tihany) are common landforms in the Bakony–Balaton Highland Volcanic Field (BBHVF, Hungary), which was active during the Late Miocene up to the Early Pleistocene. These types of monogenetic volcanoes are usually evolved in a slightly different way than their “simple” counterparts. The Kopasz-hegy Volcanic Complex (KVC) is inferred to be a vent complex, which evolved in a relatively complex way as compared to a classical “sensu stricto” monogenetic volcano. The KVC is located in the central part of the BBHVF and is one of the youngest (2.8–2.5 Ma) volcanic erosion remnants of the field. In this study, we carried out volcanic facies analysis of the eruptive products of the KVC in order to determine the possible role of changing magma fragmentation styles and/or vent migration responsible for the formation of this volcano. The evolution of the KVC started with interaction of water-saturated Late Miocene (Pannonian) mud, sand, sandstone with rising basaltic magma triggering phreatomagmatic explosive maar-diatreme forming eruptions. These explosive eruptions in the northern part of the volcanic complex took place in a N–S aligned paleovalley. As groundwater supply was depleted during volcanic activity the eruption style became dominated by more magmatic explosive-fragmentation leading to the formation of a mostly spatter-dominated scoria cone that is capping the basal maar-diatreme deposits. Subsequent vent migration along a few hundred meters long fissure still within the paleovalley caused the opening of the younger phreatomagmatic southern vent adjacent to the already established northern maar. This paper describes how change in eruption styles together with lateral migration of the volcanism forms an amalgamated vent complex.